标题是完全不说明的，输入是双值的，我想添加/减去最小的可能。 >

如果你的编译器实现了C99的数学函数/ C ++ 11，你可以使用 nextafter ：

  #include< cfloat> // DBL_MAX 
 #include< cmath> // std :: nextafter 

 double x = 0.1; 

 //在DBL_MAX的方向上x后的可表示数字
 double xPlusSmallest = std :: nextafter（x，DBL_MAX）; 
 

即使你的编译器不支持它，它可能有一个内在的。 （例如，MSVC自2005年以来已经有 _nextafter ）。

不支持它，但Boost可以使用，您可以这样做：

  #include< boost / math / special_functions /next.hpp> // boost :: float_next 

 double x = 0.1; 

 //下一个可表示的数字x 
 double xPlusSmallest = boost :: math :: float_next（x）; 
 

这相当于这个（模拟C99）：

  #include< boost / math / special_functions / next.hpp> // boost :: nextafter 
 #include< cfloat> // DBL_MAX 

 double x = 0.1; 

 //在DBL_MAX方向的x后面的下一个可表示数字
 double xPlusSmallest = boost :: math :: nextafter（x，DBL_MAX）; 
 

如果这些都不适合你，你只需要打开Boost标题，复制它。

The title is quite self-explanatory, input is given double value, and I want to add/substract the smallest amount possible.

If your compiler implements C99's math functions/C++11, you can use the nextafter:

#include <cfloat> // DBL_MAX
#include <cmath> // std::nextafter

double x = 0.1;

// next representable number after x in the direction of DBL_MAX
double xPlusSmallest = std::nextafter(x, DBL_MAX); 

Even if your compiler doesn't support it, it probably has an instrinsic for it. (MSVC has had _nextafter since 2005, for example. GCC probably implements it as standard.)

If your compiler doesn't support it but Boost is available to you, you can do this:

#include <boost/math/special_functions/next.hpp> // boost::float_next

double x = 0.1;

// next representable number after x
double xPlusSmallest = boost::math::float_next(x); 

Which is equivalent to this (emulating C99):

#include <boost/math/special_functions/next.hpp> // boost::nextafter
#include <cfloat> // DBL_MAX

double x = 0.1;

// next representable number after x in the direction of DBL_MAX
double xPlusSmallest = boost::math::nextafter(x, DBL_MAX); 

And if none of those work for you, you'll just have to crack open the Boost header and copy it.